One device of the prior art is composed of a turnbuckle in which each of the two pieces to be joined is equipped with a threaded shank, these threaded shanks being coaxial and of opposite pitch and joined by a bush comprising two opposite inside threads in which the threaded shanks engage.
The rotation of the bush in one direction or the other brings together or moves apart the two pieces, which can be held in the required position by two lock nuts. This is a clumsy system which is suitable for assemblies which do not need frequent adjustments.
There are also turnbuckles of the single tension element type, i.e., comprising only a single screw adjustment element. These devices, however, are of complex structure, notably as regards the connection between the shank, the nut and the body.